Nanocellular or nanoporous polymer foams are particularly good thermal insulation materials on the basis of theoretical considerations. This is because the internal dimensions of nanofoams are of the order of the mean free path length of a gas molecule. The gas contribution to heat transfer can be reduced in this way. Polyurethanes are a group of polymers which are frequently used in thermal insulation.
Polyurethane foams are produced by reacting a polyol component, which also contains a blowing agent, with an isocyanate. The reaction of isocyanate with water forms carbon dioxide, which also acts as a blowing agent.
The decisive step for foam formation, and hence for the later cell size of the cured foam, is the nucleation provided by blowing agents, since every cell in the foam has been formed from a gas bubble. A relevant observation here is that, after nucleation, no new gas bubbles are generally produced, but instead blowing agent diffuses into existing gas bubbles.
Addition of stabilizers promotes the emulsification of the various components, influences nucleation and prevents coalescence of growing gas bubbles. They also influence cell opening. In open-cell foams, the membranes of the growing pores are opened and the struts of the pores are left standing.
One possible approach is to emulsify a supercritical blowing agent into the reaction mixture and then to cure the foam after reducing the pressure. The POSME method (principle of supercritical micro emulsion expansion) is known as a variant thereof. The blowing agent is present therein in the form of a microemulsion. Microemulsions form under certain conditions which depend inter alia on the concentration of emulsifiers and on the temperature. Microemulsions are notable for their stability and for the fact that the nonpolar phase, i.e., the blowing agent in this case, can be present within the polar phase in very small droplets. The diameters of such droplets can range from 1 to 100 nanometers,
DE 102 60 815 A 1 discloses foamed material and a process for producing the foamed material. Foamed material comprising foam bubbles in nanosize is supposed to be produced without having to surmount the energy barrier typical of phase conversions and nucleus-forming processes. An associated goal is to produce, in a controllable manner, a foamed material that has a numeric density of foam bubbles between 1012 and 1018 per cm3 and also an average diameter for the foam bubbles of between 10 nm and 10 μm. The foundation is the dispersion of a second fluid in the form of pools in a matrix of a first fluid. A reaction space contains the first fluid as a matrix and the second fluid in pools. A change in pressure and/or temperature is used to convert the second fluid into a near-critical or supercritical state with a density close to that of a liquid. The second fluid is therefore fully or almost fully in the form of pools which have a uniform distribution in the entire first fluid. Depressurization causes the second fluid to revert to a state of gaseous density, while the pools inflate into foam bubbles of nanometer size. No energy barrier has to be surmounted, nor do the blowing agent molecules have to diffuse to the expanding bubbles.
Any polymerizable substance is said to be generally useful as first fluid. However, express mention is only made of acrylamide, which polymerizes to give polyacrylamide, and melamine, which polymerizes to give melamine resin. The second fluid should be selected from a group of materials which comprises hydrocarbons such as methane or ethane, alkanols, (hydro)chlorofluorocarbons or CO2. A further material used is an amphiphilic material that should have at least one block with affinity for the first fluid and at least one block with affinity for the second fluid.
WO 2012/146568 discloses a polyurethane foam obtainable from the reaction of a mixture comprising A) an isocyanate-reactive compound; B) a blowing agent selected from the group comprising linear, branched or cyclic C1- to C6-hydrocarbons, linear, branched or cyclic C1- to C6-(hydro)fluorocarbons, N2, O2, argon and/or CO2, wherein the blowing agent B) is in the supercritical or near-critical state; and C) a polyisocyanate.
The isocyanate-reactive compound A) comprises a hydrophobic part and has a hydrophilic part and an average hydroxyl functionality of greater than 1. The hydrophobic part comprises a saturated or unsaturated hydrocarbon chain having at least six carbon atoms and the hydrophilic part comprises alkylene oxide units and/or ester units.